The structure stability of nanometric-Ni (n-Ni) produced by Vale Inco Ltd. Canada as a catalytic additive for MgH2 has been investigated. Each n-Ni filament is composed of nearly spherical interconnected particles having a mean diameter of 42 +/- 16 nm. After ball milling of the MgH2 + 5 wt.%n-Ni mixture for 15 min the n-Ni particles are found to be uniformly embedded within the particles of MgH2 and at their surfaces. Neither during ball milling of the MgH2 + 5 wt.%n-Ni mixture nor its first decomposition at temperatures of 300, 325, 350 and 375 degrees C the elemental n-Ni reacts with the elemental Mg to form the Mg2Ni intermetallic phase (and eventually the Mg2NiH4 hydride). The n-Ni additive acts as a strong catalyst accelerating the kinetics of desorption. From the Arrhenius and Johnson-Mehl-Avrami-Kolmogorov theory the activation energy for the first desorption is determined to be similar to 94 kJ/mol. After cycling at 300 degrees C the activation energy for desorption is determined to be similar to 99 kJ/mol. This is much lower than similar to 160 kJ/mol observed for the undoped and ball milled MgH2. During cycling at 275 and 300 degrees C the n-Ni additive is converted into Mg2Ni (Mg2NiH4). The newly formed Mg2NiH4 has a nanosized grain on the order of 20 nm. Its catalytic potency seems to be similar to its n-Ni precursor. The formation of Mg2Ni (Mg2NiH4) may be one of the factors responsible for the systematic decrease of hydrogen capacity observed upon cycling at 275 and 300 degrees C. (c) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.